1. Field of the Invention
This invention relates generally to measurement and data acquisition systems and, more particularly, to an improved method and apparatus for calibrating analog-to-digital systems.
2. Description of the Related Art
Scientists and engineers often use measurement systems to perform a variety of functions, including measurement of a physical phenomena or unit under test (UUT), test and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples.
A typical measurement system comprises a computer system with a measurement device or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a card or board plugged into one of the I/O slots of the computer system, or a card or board plugged into a chassis, or an external device. For example, in a common measurement system configuration, the measurement hardware is coupled to the computer system through a PCI bus, PXI (PCI extensions for Instrumentation) bus, a GPIB (General-Purpose Interface Bus), a VXI (VME extensions for Instrumentation) bus, a serial port, parallel port, or Ethernet port of the computer system. Optionally, the measurement system includes signal conditioning devices which receive the field signals and condition the signals to be acquired.
A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a unit under test.
Measurement systems, which may also be generally referred to as data acquisition systems, may include the process of converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. PC-based measurement and data acquisition (DAQ) systems and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor.
In a measurement or data acquisition process, analog signals may be received by a digitizer, which may reside in a DAQ device or instrumentation device. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an Analog-to-Digital Converter (ADC), and transmitted to a computer system for storage and/or analysis. When a measurement system generates an analog output, the computer system may generate digital signals that are provided to one or more digital to analog converters (DACs) in the DAQ device. The DACs convert the digital signal to an analog output signal that is used, e.g., to stimulate a UUT.
Typically, measurement devices, such as data acquisition (DAQ) devices, digital multi meters (DMMs), and digital storage oscilloscopes (DSOs) need to be adjusted to provide accurate measurements of DC input signals via a method called DC voltage calibration. A basic form of calibration is for the user to supply the device with a known voltage and manually make adjustments to the device (for example, turn a trim pot or calculate mathematical correction terms). If the device has multiple input ranges, the user may need to follow a procedure for dialing in several voltages and manually adjusting each range.
More advanced multi-range devices may require the user to just provide a single voltage, which may be scaled internally by the device to calibrate its various ranges. Some devices may have an onboard precision reference so that they may be calibrated without any external connections (typically referred to as “self-calibration”). However, even these devices may occasionally need an “external calibration” procedure, where the internal reference itself may be adjusted and brought back in line with traceable voltage standards.
In prior methods of calibration, the calibration signal may simply be a very accurate DC voltage which the device uses as a “measuring stick” to make adjustments against. The drawback is that readings may be sensitive to the device's local nonlinearities (such as ADC quantization) as well as to large signal nonlinearities (such as ‘S’ shapes), both of which may make it difficult to determine the true “gain” and “offset” of a measurement device. This may be particularly a problem for devices such as DAQ boards where quantization errors may be significant or even dominant.
Additionally, scaling a precision reference to accommodate multiple input ranges may also be challenging. One of the most common methods is to divide down the precision reference using a precision resistive divider, whose accuracy is limited by the initial matching of the resistors and/or by the relative drift of the resistors. Other devices use a pulse-width modulation (PWM) circuit to scale a precision reference for various ranges. A PWM circuit is typically a circuit that accepts a precision reference and a digital pulse train as inputs, and provides as output a signal with an average value that is proportional to the duty cycle of the pulse train input. In these devices, the PWM circuit typically outputs a DC waveform with no ripple. These circuits are often accurately described as PWM digital-to-analog converters (PWM DACs) and are excellent during calibration both for their flexibility and for their good linearity.